A variety of chemical surface treatments are used in the paper making industry to impart various properties to the finished paper. Resistance to oil penetration is a particularly desirable property for paper products intended for use as packaging for fatty or greasy materials, for example, fatty or greasy food products. Typically, fluorocarbon compounds are employed as surface sizes or coatings to impart oil penetration resistance. A comprehensive discussion of the use of fluorocarbon compounds, both as internal as well as surface sizes, to impart oil resistance to paper is found in TAPPI Monograph No. 33 by Rengel and Young, pp. 170-188 (1971).
Fluorocarbon surface sizes, though effective, are quite expensive and available from only a few manufacturers, e.g., the proprietary fluorocarbon sizes sold by 3M Company under the "Scotchban" trademark.
Copending, commonly owned U.S. application Ser. No. 07/270,553, filed Nov. 14, 1988 discloses that certain melamine based compositions when applied to paper as surface coatings in the form of nonionic aqueous emulsions, impart, inter alia, oil penetration resistance to the paper. These melamine based compositions can be used alone or can be used to replace a substantial portion of the considerably more expensive fluorocarbon sizes, while still maintaining the high level of oil resistance obtained from the fluorocarbon sizes alone.
It has now been found that when the surface sizing compositions of U.S. application Ser. No. 07/270,553 are applied in the form of cationic aqueous emulsions, as opposed to nonionic or anionic emulsions, that improved oil penetration resistance is imparted to the sized or coated paper.